1. Field of the Invention
The present invention relates to a technique of correcting registration in an image forming apparatus using an electrostatic recording system, an electrophotographic recording system or the like.
2. Description of the Related Art
In a color image forming apparatus, a color image is formed by superposing images of different colors on one another. This image forming requires forming the plurality of images of different colors at correct positions without relative misalignment.
In a color image forming apparatus using an intermediate transfer belt, there is a possibility of expansion and contraction of an image according to the period of rotation of a drive roller for driving the intermediate transfer belt due to driving nonuniformity of the drive roller or the like. A technique of reducing the influence of image expansion and contraction due to driving nonuniformity of a drive roller has been proposed (Japanese Patent Laid-Open No. 2002-014507). According to the proposition, a pattern image used for correction of registration is formed at a plurality of positions to cancel the influence of image expansion/contraction. A plurality of groups of data on the formed pattern images are obtained by reading the images. An average of the amounts of misalignment is obtained from the plurality of data groups obtained. This average is a value at which the influence of image expansion and contraction is cancelled. Therefore, if correction processing is performed by using the average, the correction accuracy is improved.
For example, ten pattern images are formed with respect to nine periods of a drive roller 32, as shown in FIG. 13. Sampling times are thereby distributed uniformly in one period of the drive roller, as shown in FIG. 14. Thus, the influence of image expansion and contraction is suitably cancelled. More specifically, “misalignments” due to image expansion and contraction are cancelled as between phases p1 and p6, phases p2 and p7, phases p3 and p8, phases p4 and p9, and phases p5 and p10.
The method of increasing sample values of the pattern images and performing averaging process as described above is effective in reducing the influence of image extraction and contraction. However, if sample values are excessively increased, a problem arises that the correction processing time is increased and the consumption of toners for forming the pattern images is increased. Further, an increased load is imposed on the cleaning member for cleaning of the formed pattern images. Therefore it is desirable to minimize the number of pattern images.
It is preferable to form the pattern image in a space area (sheet-to-sheet space) arisen between the area in which the kth (k: an integer) output image is formed and the area in which (k+1)th output image is formed in order to correct registration while maintaining the desired productivity. However, if the pattern image is formed in the sheet-to-sheet space, there is a possibility of a restriction on the length of the image forming medium in the medium conveying direction, the width of the sheet-to-sheet space (the length in the conveying direction) or the frequency with which the pattern image is repeatedly formed, depending image forming modes. Needless to say, unless the pattern image is formed at a suitable position according to the cycle of rotation of the drive roller, the bad influence of the drive roller cannot be cancelled and an improvement in correction accuracy cannot be expected.